It is known to equip a motor vehicle with a so-called head up display system. Such a system is placed in the field of vision of the motor vehicle driver and displays information relating to the status of the motor vehicle, the traffic or other.
In order not to interfere with driving, it is necessary that the brightness of the projected image is adapted to the ambient brightness. In particular, it is necessary that the optical power of the projected image is greatly different during a daytime journey and a journey at night or when passing through a tunnel.
With head up displays using light emitting diodes, it has already been proposed to vary the brightness of the projected image by controlling the supply current of the diodes. Such devices allow a satisfactory adaptation of the brightness. However, their daytime brightness remains inadequate.
In order to compensate for this defect, displays are known which use light sources of the laser type. However, the adaptation of the brightness provided in such a case by controlling the amount of current supplying the sources does not make it possible to sufficiently reduce the optical power of the image. By way of example, if it is considered that it is desired to have an optical power of the order of 10000 Cd/m2 in daytime driving condition and of the order of 5 Cd/m2 in night driving conditions, it is observed that it is necessary to reduce the brightness by a factor of 2000 when changing from one of the driving conditions to the other.
Research conducted by the applicant on adapting the brightness of the projected image by controlling the current supply of the light sources, for example using pulse width modulation technologies or control of the amplitude of the current used, shows that these technologies have limits. It is notably necessary to have a minimum pulse duration and a minimum amplitude of the current for the linear operation of the diodes.